Teaching Assistant: Roi Yehoshua
roiyeho@gmail.com
Agenda
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Create a 3D model of our robot
URDF and Xacro files
Joint State Publisher
Watching the 3D model on rviz
(C)2014 Roi Yehoshua
URDF
• Unified Robot Description Format (URDF) is an
XML format for representing a robot model
• In this file you define:
– Kinematic and dynamic description of the robot
– Visual representation of the robot
– Collision model of the robot
• A list of robots described by URDF files can be
found here: http://wiki.ros.org/urdf/Examples
(C)2014 Roi Yehoshua
URDF Format
• The description of a robot consists of a set of link
(part) elements, and a set of joint elements
connecting the links together
• A typical robot description looks like this:
<robot name="pr2">
<link> ... </link>
<link> ... </link>
<link> ... </link>
<joint> .... </joint>
<joint> .... </joint>
<joint> .... </joint>
</robot>
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URDF Turorials
• We’re going to build a visual model of a robot
that vaguely looks like R2D2
• All of the robot models and launch files
mentioned here can be found in the urdf_tutorial
package (already installed with ROS)
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One Shape
• First, we’re just going to create one simple shape
• Look at 01-myfirst.urdf:
<?xml version="1.0"?>
<robot name="myfirst">
<link name="base_link">
<visual>
<geometry>
<cylinder length="0.6" radius="0.2"/>
</geometry>
</visual>
</link>
</robot>
• This creates a robot with the name myfirst, that contains
only one link, whose visual component is just a cylinder
0.6 meters long with a 0.2 meter radius.
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Watching the 3D Model in rviz
display.launch (in urdf_tutorial package)
<launch>
<arg name="model" />
<arg name="gui" default="False" />
<param name="robot_description" textfile="$(arg model)" />
<param name="use_gui" value="$(arg gui)"/>
<node name="joint_state_publisher" pkg="joint_state_publisher"
type="joint_state_publisher"></node>
<node name="robot_state_publisher" pkg="robot_state_publisher"
type="state_publisher" />
<node name="rviz" pkg="rviz" type="rviz" args="-d $(find
urdf_tutorial)/urdf.rviz" />
</launch>
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Watching the 3D Model in rviz
• This launch file does three things:
– Loads the specified model into the parameter server
– Runs nodes to publish the JointState and transforms
– Starts rviz with a configuration file
• To run the launch file type
– If you are in the same directory where the urdf file is:
$ roslaunch urdf_tutorial display.launch model:=01-myfirst.urdf
– If you are in a different directory:
$ roslaunch urdf_tutorial display.launch model:='$(find
urdf_tutorial)/urdf/01-myfirst.urdf'
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Watching the 3D Model in rviz
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Multiple Links
• In order to add more link elements to the urdf,
you need to define joints
• The joint is connects a parent and a child link
• There are different types of joints
• We’ll start with fixed joints
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02-multipleshapes.urdf
<?xml version="1.0"?>
<robot name="multipleshapes">
<link name="base_link">
<visual>
<geometry>
<cylinder length="0.6" radius="0.2"/>
</geometry>
</visual>
</link>
<link name="right_leg">
<visual>
<geometry>
<box size="0.6 .2 .1"/>
</geometry>
</visual>
</link>
<joint name="base_to_right_leg" type="fixed">
<parent link="base_link"/>
<child link="right_leg"/>
</joint>
</robot>
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Multiple Links
$ roslaunch urdf_tutorial display.launch model:=02-multipleshapes.urdf
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Origins
• Both of the shapes overlap with each other,
because they share the same origin
• If we want them not to overlap we must define
more origins
• Both the link and the joint elements may contain
origin tags
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Link’s Origin Tag
• <origin> - defines where the center of the visual
element should be relative to its origin
– defaults to identity if not specified
• Properties:
– xyz – represents the offset
• defaults to zero vector
– rpy - represents the rotation (roll, pitch, yaw)
• defaults to identity
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Joint’s Origin Tag
• <origin> - This is the transform from the parent link
to the child link. The joint is located at the origin of
the child link
– defaults to identity if not specified)
• Has the same properties like the link’s origin tag (xyz
and rpy)
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03-origins.urdf
<?xml version="1.0"?>
<robot name="origins">
<link name="base_link">
<visual>
<geometry>
<cylinder length="0.6" radius="0.2"/>
</geometry>
</visual>
</link>
<link name="right_leg">
<visual>
<geometry>
<box size="0.6 .2 .1"/>
</geometry>
<origin rpy="0 1.57075 0" xyz="0 0 -0.3"/>
</visual>
</link>
<joint name="base_to_right_leg" type="fixed">
<parent link="base_link"/>
<child link="right_leg"/>
<origin xyz="0.22 0 .25"/>
</joint>
</robot>
(C)2014 Roi Yehoshua
Origins
• The joint’s origin is defined in terms of the parent’s
reference frame.
– In this case the origin for the child link will be .22 meters
in the x direction (right) and .25 meters in the z direction
(up)
• The leg’s visual origin defines where the center of
the visual element should be relative to its origin
– Here we want the leg to attach at the top, so we offset
the origin down by setting the z offset to be -.3 meters
– And since we want the long part of the leg to be parallel
to the z axis, we rotate the visual part PI/2 around the Y
axis
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Examining the Model
$ roslaunch urdf_tutorial display.launch model:=03-origins.urdf
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Material Girl
• That’s very cute, but not all robots are red…
• The material tag of the visual element allows you
to specify the material used by the link element
• Properties:
– name – name of the material. Can refer to a
previously defined material.
– <color> (optional)
• rgba The color of a material specified by set of four
numbers representing red/green/blue/alpha, each in the
range of [0,1].
– <texture> (optional)
• The texture of a material is specified by a filename
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04-materials.urdf (1)
<?xml version="1.0"?>
<robot name="materials">
<link name="base_link">
<visual>
<geometry>
<cylinder length="0.6" radius="0.2"/>
</geometry>
<material name="blue">
<color rgba="0 0 .8 1"/>
</material>
</visual>
</link>
<link name="right_leg">
<visual>
<geometry>
<box size="0.6 .2 .1"/>
</geometry>
<origin rpy="0 1.57075 0" xyz="0 0 -0.3"/>
<material name="white">
<color rgba="1 1 1 1"/>
</material>
</visual>
</link>
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04-materials.urdf (2)
<joint name="base_to_right_leg" type="fixed">
<parent link="base_link"/>
<child link="right_leg"/>
<origin xyz="0.22 0 .25"/>
</joint>
<link name="left_leg">
<visual>
<geometry>
<box size="0.6 .2 .1"/>
</geometry>
<origin rpy="0 1.57075 0" xyz="0 0 -0.3"/>
<material name="white"/>
</visual>
</link>
<joint name="base_to_left_leg" type="fixed">
<parent link="base_link"/>
<child link="left_leg"/>
<origin xyz="-0.22 0 .25"/>
</joint>
</robot>
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Examining the Model
$ roslaunch urdf_tutorial display.launch model:=04-materials.urdf
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Loading Meshes to Our Model
• Mesh files allow you to define more realistic
elements than just using basic geometric
objects/blocks
• It is possible to load meshes generated by us or to
use meshes of other models.
• For our model we will use the PR2's gripper
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Loading Meshes to Our Model
<link name="left_gripper">
<visual>
<origin rpy="0.0 0 0" xyz="0 0 0"/>
<geometry>
<mesh filename="package://pr2_description/meshes/gripper_v0/l_finger.dae"/>
</geometry>
</visual>
</link>
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pr2_description
• To borrow the meshes from PR2, you’ll need to
install the pr2_description package:
$ sudo apt-get install ros-hydro-pr2-description
• The package contains robot description files for PR2,
organized into subdirectories as follows:
– urdf/ contains urdf descriptions of various parts of the
PR2 (arm, torso, etc.)
– robots/ contains urdf descriptions of the full robot, that
refer to the macros in urdf/
– gazebo/ contains urdf descriptions of simulated PR2
components, like the simulated battery controller
– meshes/ contains mesh files (.stl,.dae) for visualization
and collision properties
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Example of a Collada File
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Finishing the Model
• Let's finish the design by adding some parts: left
and right bases, four wheels and an arm with a
gripper
• The final urdf file is located at 05-visual.urdf
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Finishing the Model
$ roslaunch urdf_tutorial display.launch model:=05-visual.urdf
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Building a Movable Robot Model
• To convert the model into a robot that can
actually move, the only thing you have to do is
take care of the type of the joints it uses
• In the previous model, all of the joints were fixed
• Now we are going to explore different types of
flexible joints
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Joint Types
• revolute - a hinge joint that rotates along the axis and has
a limited range specified by the upper and lower limits
• continuous - a continuous hinge joint that rotates around
the axis and has no upper and lower limits
• prismatic - a sliding joint that slides along the axis, and has
a limited range specified by the upper and lower limits
• fixed - This is not really a joint because it cannot move
• floating - This joint allows motion for all 6 degrees of
freedom
• planar - This joint allows motion in a plane perpendicular
to the axis
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Joint Types
• Compare to joint types in the human body:
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The Head
<joint name="head_swivel" type="continuous">
<parent link="base_link"/>
<child link="head"/>
<axis xyz="0 0 1"/>
<origin xyz="0 0 0.3"/>
</joint>
• The connection between the body and the head is a
continuous joint, meaning that it can take on any
angle from -∞ to +∞
• The only additional information we have to add is
the axis of rotation (the <axis> tag), which specifies a
vector around which the head will rotate.
– the vector "0 0 1" means it will rotate around the z axis
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The Wheels
<joint name="right_front_wheel_joint" type="continuous">
<axis xyz="0 0 1"/>
<parent link="right_base"/>
<child link="right_front_wheel"/>
<origin rpy="0 1.57075 0" xyz="0 0.133333333333 -0.085"/>
<limit effort="100" velocity="100"/>
<dynamics damping="0.0" friction="0.0"/>
</joint>
• The wheels are also modeled as continuous joints,
so that they can roll in both directions forever
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The Gripper
<joint name="left_gripper_joint" type="revolute">
<axis xyz="0 0 1"/>
<limit effort="1000.0" lower="0.0" upper="0.548" velocity="0.5"/>
<origin rpy="0 0 0" xyz="0.2 0.01 0"/>
<parent link="gripper_pole"/>
<child link="left_gripper"/>
</joint>
• The right and the left gripper joints are modeled as
revolute joints
• This means that they rotate in the same way that the
continuous joints do, but they have strict limits
• Hence, we must include the <limit> tag specifying the
upper and lower limits of the joint (in radians)
• We also must specify a maximum velocity and effort for
this joint
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The Gripper Arm
<joint name="gripper_extension" type="prismatic">
<parent link="base_link"/>
<child link="gripper_pole"/>
<limit effort="1000.0" lower="-0.38" upper="0" velocity="0.5"/>
<origin rpy="0 0 1.57075" xyz="0 0.19 .2"/>
</joint>
• The gripper arm is modeled as a prismatic joint
– This means that it moves along an axis, not around it
• This translational movement is what allows our
robot model to extend and retract its gripper arm
• The limits of the prismatic arm are specified in the
same way as a revolute joint, except that the units
are meters, not radians
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Building a Movable Robot Model
• 06_flexible.urdf is the new urdf with flexible
joints
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Joint State Publisher
• http://wiki.ros.org/joint_state_publisher
• This package contains a tool for setting and
publishing joint state values for a given URDF
• Publishes sensor_msgs/JointState messages for
all the non-fixed joints of the robot
• Can be used in conjunction with the
robot_state_publisher node to also publish
transforms for all joint states
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Joint State Publisher GUI
• To visualize and control this model, type:
$ roslaunch urdf_tutorial display.launch model:=06-flexible.urdf gui:=True
• This will also pop up a GUI that allows you to
control the values of all the non-fixed joints
• Play with the model some and see how it moves
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Joint State Publisher GUI
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Adding Physical and Collision Properties
• If you want to simulate the robot on Gazebo or any
other simulation software, it is necessary to add
physical and collision properties
• We need to set on every link the dimension of the
geometry to calculate the possible collisions, the
weight that will give us the inertia, and so on.
• 07-physics.urdf is the new URDF with the physical
properties
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Collision
• The collision element is a direct subelement of the
link object, at the same level as the visual tag
• The collision element defines its shape the same
way the visual element does, with a geometry tag
• The format for the geometry tag is exactly the same
here as with the visual.
• You can also specify an origin in the same way as a
subelement of the collision tag (as with the visual)
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Collision Element Example
<link name="base_link">
<visual>
<geometry>
<cylinder length="0.6" radius="0.2"/>
</geometry>
<material name="blue">
<color rgba="0 0 .8 1"/>
</material>
</visual>
<collision>
<geometry>
<cylinder length="0.6" radius="0.2"/>
</geometry>
</collision>
</link>
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Collision
• Usually the collision geometry is defined exactly the
same as the visual geometry
• There are two main cases where you wouldn’t:
– Quicker Processing - Doing collision detection for two
meshes is a lot more computational complex than for two
simple geometries. Hence, you may want to replace the
meshes with simpler geometries in the collision element.
– Safe Zones - You may want to restrict movement close to
sensitive equipment. For instance, if we didn’t want
anything to collide with R2D2’s head, we might define the
collision geometry to be a cylinder encasing his head to
prevent anything from getting to near his head.
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Inertia
• Inertia is the resistance of any physical object to any
change in its state of motion, including changes to its
speed and direction
• It is the tendency of objects to keep moving in a straight
line at constant velocity
• The inertia tensor depends on both the mass and the
distribution of mass of the object
– A good explanation on intertia tensor can be found here
• The 3x3 rotational inertia matrix is specified with the
inertia element
ixx
ixy
ixz
ixy
iyy
iyz
ixz
iyz
izz
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Inertia Example
<link name="base_link">
<visual>
<geometry>
<cylinder length="0.6" radius="0.2"/>
</geometry>
<material name="blue">
<color rgba="0 0 .8 1"/>
</material>
</visual>
<collision>
<geometry>
<cylinder length="0.6" radius="0.2"/>
</geometry>
</collision>
<inertial>
<mass value="10"/>
<inertia ixx="1.0" ixy="0.0" ixz="0.0" iyy="1.0" iyz="0.0" izz="1.0"/>
</inertial>
</link>
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Link Element Summary
• The link element describes a rigid body with an
inertia, visual and collision features
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Joint Element Summary
• The joint element describes the kinematics and
dynamics of the joint and also specifies the
safety limits of the joint
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Xacro
• Notice the size of the 07-physics.urdf file
• It has 414 lines of code to define our robot (!)
• Imagine if you start to add cameras and other
geometries, the file will start to increase and its
maintenance will become even more complicated
• Xacro helps to reduce the overall size of the URDF
file and makes it easier to read and maintain.
• It also allows us to create modules and reuse them
to create repeated structures such as several arms
or legs
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Using Xacro
• As its name implies, xacro is a macro language.
• The xacro program runs all of the macros and
outputs the result as a urdf file
• Typical usage:
$ rosrun xacro xacro.py model.xacro > model.urdf
• You can also automatically generate the urdf in a
launch file:
<param name="robot_description" command="$(find xacro)/xacro.py
'$(find pr2_description)/robots/pr2.urdf.xacro'" />
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Using Xacro
• In the xacro file, you must specify a namespace
in order for the file to parse properly.
• For example, these are the first two lines of a
valid xacro file:
<?xml version="1.0"?>
<robot xmlns:xacro="http://www.ros.org/wiki/xacro" name="robot1_xacro">
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Constants
• We can use xacro to declare constant values
• This way we can avoid putting the same value in a
lot of lines and will make it easier to maintain
changes.
• You can define constants using <xacro:property> tag,
usually located at the top of the file
• Then to use the constant’s value, you can write
${name_of_variable}
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Constants Example
<xacro:property name="width" value=".2" />
<xacro:property name="bodylen" value=".6" />
<link name="base_link">
<visual>
<geometry>
<cylinder radius="${width}" length="${bodylen}"/>
</geometry>
<material name="blue">
<color rgba="0 0 .8 1"/>
</material>
</visual>
<collision>
<geometry>
<cylinder radius="${width}" length="${bodylen}"/>
</geometry>
</collision>
</link>
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Math
• You can build up arbitrarily complex expressions in
the ${} construct using the four basic operations (+,,*,/), the unary minus, and parenthesis.
• Exponentiation and modulus are not supported.
• Examples:
<cylinder radius="${wheeldiam/2}" length=".1"/>
<origin xyz="${reflect*(width+.02)} 0 .25" />
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Macros
• Macros are the most useful component of the
xacro package
• Example for a simple macro:
<xacro:macro name="default_origin">
<origin xyz="0 0 0" rpy="0 0 0"/>
</xacro:macro>
<xacro:default_origin />
• This code will generate the following:
<origin xyz="0 0 0" rpy="0 0 0"/>
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Parameterized Macros
• You can also parameterize macros so that they don’t
generate the same exact text every time
• When combined with the math functionality, this is
even more powerful.
• An inertia macro example:
<xacro:macro name="default_inertial" params="mass">
<inertial>
<mass value="${mass}" />
<inertia ixx="1.0" ixy="0.0" ixz="0.0"
iyy="1.0" iyz="0.0"
izz="1.0" />
</inertial>
</xacro:macro>
<xacro:default_inertial mass="10"/>
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Example of a Leg Macro
<xacro:macro name="leg" params="prefix reflect">
<link name="${prefix}_leg">
<visual>
<geometry>
<box size="${leglen} .2 .1"/>
</geometry>
<origin xyz="0 0 -${leglen/2}" rpy="0 ${pi/2} 0"/>
<material name="white">
<color rgba="1 1 1 1"/>
</material>
</visual>
<collision>
<geometry>
<box size="${leglen} .2 .1"/>
</geometry>
<origin xyz="0 0 -${leglen/2}" rpy="0 ${pi/2} 0"/>
</collision>
<xacro:default_inertial mass="10"/>
</link>
<joint name="base_to_${prefix}_leg" type="fixed">
<parent link="base_link"/>
<child link="${prefix}_leg"/>
<origin xyz="${reflect*(width+.02)} 0 .25" />
</joint>
<!-- A bunch of stuff cut -->
</xacro:macro>
<xacro:leg prefix="right" reflect="1" />
<xacro:leg prefix="left" reflect="-1" />
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Examining the Xacro
• Here is the xacro used in the R2D2 model
• Number of lines was reduced from 414 to 236
• To see the model generated by the xacro file, run:
$ roslaunch urdf_tutorial xacrodisplay.launch model:=08-macroed.urdf.xacro
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Examining the Xacro
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URDF Verification
• The robot model stack includes the command line
tool check_urdf
• It's called with a single command line argument
naming a file
• It attempts to parse the file as a URDF description,
and either prints a description of the resulting
kinematic chain, or an error message
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URDF Verification
• For example, to run this tool on the R2D2 urdf:
$ check_urdf 07-physics.urdf
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URDF Visualization
• To get a graphviz diagram of your urdf file, type:
$ cp 07-physics.urdf /tmp
$ urdf_to_graphiz /tmp/07-physics.urdf
$ evince /tmp/physics.pdf
• The result is a file called physics.pdf that looks
something like this:
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URDF Visualization
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PR2 URDF
• To see the PR2 URDF graphically, first create the
urdf file by running:
$ rosrun xacro xacro.py `rospack find
pr2_description`/robots/pr2.urdf.xacro > /tmp/pr2.urdf
• Then run the urdf_to_graphiz tool:
$ rosrun urdfdom urdf_to_graphiz /tmp/pr2.urdf
$ evince /tmp/pr2.urdf
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PR2 URDF
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Watch PR2 in rviz
• To watch PR2’s urdf in rviz, follow these steps:
$ cd /tmp
$ roslaunch urdf_tutorial display.launch model:=pr2.urdf gui:=True
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Homework (not for submission)
• Create a URDF model for the following robot
– The robot contains 4 links (base, left and right wheels and a
caster) and 3 joints that connect the wheels and the caster to
the base
– Attach to the robot's body a mesh taken from the Pioneer
robot's model (model://pioneer2dx/meshes/chassis.dae)
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